Method of precipitation-hardening a nickel alloy

ABSTRACT

The Application relates to a precipitation hardening alloy which has a 0.2% proof stress of at least 500 N/mm 2  and a high resistance to corrosion in highly aggressive sour gas media. The alloy consists of 43 to 51% nickel, 19 to 24% chromium, 4.5 to 7.5% molybdenum, 0.4 to 2.5% copper, 0.3 to 1.8% aluminium and 0.9 to 2.2% titanium, residue iron. Heat treatment processes are described which allow the establishment in the alloy of high strength accompanied by satisfactory ductility.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a precipitation hardening nickel alloy having a0.2% proof stress of at least 500 N/mm² and very good resistance tocorrosion, the invention also relating to the use of said alloy for themaking of structural components required to meet the aforementioneddemands and to a process for the production of such structuralcomponents.

Very high resistance to corrosion means that the alloy and componentsmade thereof can be exposed at temperatures between room temperature and350° C. and pressures between 10 and 100 bar to solutions containingCO₂, H₂ S, chlorides and free sulfur.

Such conditions are typically found in oil and natural gas explorationand production. Structural components meeting the aforementionedconditions have hitherto been made from nickel-based materials alloyedwith chromium and molybdenum, although their 0.2% proof stress is onlyapproximately 310 to 345 N/mm². Their strength can be enhanced by coldworking, although at the same time a reduction in ductility must betolerated. Moreover, as a rule strain hardening cannot be used with verylarge cross-sections, so that in such cases precipitation hardeningmaterials must be resorted to. However, in highly aggressive sour gasconditions materials which can be given higher strengths byprecipitation hardening have inadequate resistance to corrosion, or theycontain niobium as an essential alloying element required forprecipitation hardening.

2. Description of the Prior Art

For example, J. A. Harris, T. F. Lemke, D. F. Smith and R. H. Moellerproposed a precipitation hardening nickel-based material containing 42%nickel, 21% chromium, 3% molybdenum, 2.2% copper, 2.1% titanium, 0.3%aluminium, 0.02% carbon, residue iron, which was alleged to be resistantin sour gas conditions (The Development of a Corrosion Resistant Alloyfor Sour Gas Service, CORROSION 84, Paper No. 216, National Associationof Corrosion Engineers, Houstin, Tex., 1984). However, their publishedresults show that in conditions of extreme corrosion, such as may existat greater depths, the material proposed is destroyed by stresscorrosion cracking.

Another alloy was proposed in European Patent Specification 0066361.That proposed alloy contained (in % by weigh) in addition to 45 to 55%nickel, 15 to 22% chromium, 6 to 9% molybdenum, 2.5 to 5.5% niobium, 1to 2% titanium, up to 1% aluminium, up to 0.35% carbon and 10 to 28%iron and other accompanying elements, also niobium as an alloyingcomponent essential for precipitation hardening. However,niobium-containing alloys are much less suitable for large scaleindustrial manufacture and processing than niobium-free alloys, sinceniobium-containing scrap and production wastes require a vacuuminduction furnace for remelting if appreciable losses of this expensivealloying element by burn-off are to be avoided. Moreover, higher niobiumcontents, such as those here proposed, very clearly reduce thepossibilities of hot shaping of the material. Similar disadvantages alsoapply to the alloy proposed by R. B. Frank and T. A. DeBold which have(in % by weight) 59 to 63% nickel, 19 to 22% chromium, 7 to 9.5%molybdenum, 2.75 to 4% niobium, 1 to 1.6% titanium, maximum 0.35%aluminium, maximum 0.03% carbon, residue iron (Properties of anAge-Hardenable, Corrosion-Resistant, Nickel-Base Alloy, CORROSION, 88Paper No. 75, National Association of Corrosion Engineers, Houston,Tex., 1988). Due to its high nickel content, this alloy can also beexpected to have a marked tendency towards hydrogen embrittlement insour gas conditions in the temperature range below approximately 100°C., so that in this respect it has limited utilizability.

The problem therefore exists of providing a precipitation hardeningmaterial which meets all the aforementioned requirements--i.e., has therequired strength values, adequate resistance to corrosion in highlyaggressive sour gas conditions, and requires no niobium forprecipitation hardening.

SUMMARY OF THE INVENTION

To solve this problem the invention provides a precipitation hardeningnickel alloy which is characterized by

43 to 51% nickel

19 to 24% chromium

4.5 to 7.5% molybdenum

0.4 to 2.5% copper

up to 1% manganese

up to 0.5% silicon

up to 0.02% carbon

up to 2% cobalt

0.3 to 1.8% aluminium

0.9 to 2.2% titanium

residue iron, including unavoidable impurities due to manufacture.

The nickel alloy according to the invention is suitable as a materialfor the making of structural components which must have a 0.2% proofstress of at least 500 N/mm², an elongation without necking A₅ of atleast 20%, a reduction of area after fracture of at least 25% and anabsorbed energy per cross-sectional area at room temperature of at least54 J, corresponding to at least 40 ft lbs, with ISO V specimens.

A limited composition having particularly satisfactory workabilityproperties is characterized by

46 to 51% nickel

20 to 23.5% chromium

5 to 7% molybdenum

1.5 to 2.2% copper

up to 0.8% manganese

up to 0.1% silicon

up to 0.015% carbon

up to 2% cobalt

0.4 to 0.9% aluminium

1.5 to 2.1% titanium

residue iron, including unavoidable impurities due to manufacture.

This can be used if the requirements are for a 0.2% proof stress of atleast 750 N/mm², an elongation without necking A₅ of at least 20%, areduction of area after fracture of at least 25% and an absorbed energyper cross-sectional area at room temperature of at least 54 H,corresponding to at least 40 ft lbs, with ISO V samples.

The nickel alloy is more particularly suitable as a material for themaking of structural components which are to be used in highlyaggressive sour gas conditions.

In the manufacture of structural components which must have an adequateresistance to corrosion in highly aggressive sour gas conditions and a0.2% proof stress of at least 500 N/mm², conveniently the procedure isthat ingots are produced from an alloy having

43 to 51% nickel

19 to 24% chromium

4.5 to 7.5% molybdenum

0.4 to 2.5% copper

up to 1% manganese

up to 0.5% silicon

up to 0.02% carbon

up to 2% cobalt

0.3 to 1.8% aluminium

0.9 to 2.2% titanium

residue iron, including unavoidable impurities due to manufacture.

The ingots are homogenized at 1120° C. and then hot shaped at atemperature above 1000° C., the resulting components being quenched inwater, and the hot shaped quenched components are precipitation hardenedfor 4 to 16 hours at 650° to 750° C. and then subjected to air cooling.

For ingots which must have particularly good workability properties,preferably the following alloy is used, having

46 to 51% nickel

20 to 23.5% chromium

5 to 7% molybdenum

1.5 to 2.2% copper

up to 0.8% manganese

up to 0.1% silicon

up to 0.015% carbon

up to 2% cobalt

0.4 to 0.9% aluminium

1.5 to 2.1% titanium

residue iron, including unavoidable impurities due to manufacture.

In addition to the single-stage heat treatment mentioned, the mechanicaland technological properties can be further improved by additionalprecipitation hardening steps. In that case the hot shaped, quenchedcomponents are first annealed for 4 to 10 hours at 700° to 750° C., thenfurnace-cooled in a controlled manner by 150° C. at a rate of 5° to 25°C. per hour, and finally deposited in air. Alternatively, the structuralcomponents can also be held between 730° and 750° C. for 30 minutes,then furnace-cooled to 700° C. at a rate of 5° to 25° C. per hour, andfinally cooled in a controlled manner to 580° C. at a rate of 2° to 15°C. per hour. Finally the structural components are deposited in air.

In a further variant of the manufacturing process, prior to beingquenched in water, the hot shaped components are subjected to a solutionannealing at 1150° to 1190° C. Lastly according to a possible feature ofthe invention the hot shaped solution-annealed water-quenched componentsare held for 4 to 10 hours at 700° to 750° C., then furnace-cooled by150° C. at a rate of 5° to 25° C. per hour and finally subjected tofurther air cooling.

Other details and advantages of the invention will be explained ingreater detail with reference to the following test results.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Table 1 shows the chemical composition of 7 alloys which after differentheat treatments were investigated for their mechanical properties atroom temperature (RT) and at 260° C. The results are set forth in Tables2 to 7.

From ingots weighing approximately 45 kg, following solution annealingat 1220° C., rods having a diameter of approximately 18 mm were hotforged at temperatures above 1000° C. Thereafter the rods were eitherquenched directly in water or again solution annealed and then quenchedin water. Subsequently the samples thus prepared were subjected to asingle to triple stage precipitation hardening treatment. In the firststage annealing temperatures of 730° or 750° C. and annealing times of8, 4 or 0.5 hours were used. In the case of the two-stage process thiswas followed by a controlled cooling at the rate of 15° C. per hour to600° or 580° C., while in the triple stage process first a controlledcooling at 700° C. at the rate of 5° C. per hour and then a furthercontrolled cooling to 580° C. at the rate of 15° C. per hour wereperformed before the samples were subjected to further uncontrolledcooling in air.

The results show that in all cases the required minimum values of themechanical properties were achieved and in some cases appreciablyexceeded. Furthermore, results as a whole show that the differentvariants of the heat treatment enable different values of mechanicalproperties to be achieved, something which may be advantageous foradjustment to specially required sections. For example, higherelongation values at rupture can be achieved at the expense of maximumstrength values and vice versa. Apart from this general tendency,however, it can be seen that the highest strength values are achieved ifthe hot shaped components are not yet even solution annealed, butdirectly quenched in water, while the maximum achievable strengthdepends on the total content of aluminium plus titanium.

However, the aluminium and titanium contents cannot be increased to justany extent, since in that case disadvantageous precipitation phasesoccur which cannot be prevented or compensated even by an expensive heattreatment. On the other hand, due to the numerous alternative heattreatments, within the framework of the composition according to theinvention it is always possible to obtain maximum strength values inevery case without having to allow for disadvantageous structures. Thus,the more expensive triple stage precipitation hardening treatment willbe indicated, for example, if the objective is to obtain the highestpossible strength values without a reduction of the absorbed energy percross-sectional area.

To examine resistance to stress corrosion cracking, three-point bendingsamples were tested with two different corrosive media in an autoclave.In dependence on the preceding heat treatment, the samples weresubjected to different test loads, the values 100% R_(p0).2 and also120% R_(p0).2 having been selected as reference values. The testtemperatures were 232° C. and 260° C.

The solutions A and B by which the sour gas conditions were simulatedcontained:

Solution A: 25% NaCl, 10 bar H₂ S and 50 bar CO₂

Solution B: 25% NaCl, 0.5% acetic acid, 1 g/l sulfur and 12 bar H₂ S.

Tables 8 to 13 show the results of these corrosion investigations,stating the test conditions.

It can be seen that following the test cycle of between 23 and 26 daysnone of the samples showed any rupture or any attack pointing to stresscorrosion cracking.

The alloy according to the invention therefore discloses in a novelmanner a combination of high strength and outstanding resistance inhighly aggressive sour gas media hitherto unachieved using precipitationhardening materials.

                                      TABLE 1                                     __________________________________________________________________________    Composition of the examples in % by weight                                    Alloy No.                                                                           Ni Cr Fe  Mo Mn Si Cu                                                                              C  Al Ti Al + Ti                                   __________________________________________________________________________    1     46.6                                                                             22.1                                                                             residue                                                                           7.4                                                                              0.48                                                                             0.10                                                                             2.0                                                                             0.007                                                                            0.40                                                                             1.80                                                                             2.20                                      2     49.1                                                                             20.7                                                                             "   6.0                                                                              0.49                                                                             0.05                                                                             1.8                                                                             0.008                                                                            0.62                                                                             1.73                                                                             2.35                                      3     44.9                                                                             23.3                                                                             "   7.1                                                                              0.52                                                                             0.11                                                                             2.0                                                                             0.014                                                                            0.53                                                                             2.01                                                                             2.54                                      4     47.4                                                                             22.3                                                                             "   6.1                                                                              0.49                                                                             0.05                                                                             1.8                                                                             0.011                                                                            0.64                                                                             1.95                                                                             2.59                                      5     45.0                                                                             23.3                                                                             "   7.1                                                                              0.49                                                                             0.10                                                                             2.0                                                                             0.015                                                                            1.01                                                                             1.97                                                                             2.98                                      6     45.7                                                                             23.1                                                                             "   7.0                                                                              0.48                                                                             0.08                                                                             2.0                                                                             0.011                                                                            1.10                                                                             1.90                                                                             3.00                                      7     45.3                                                                             23.0                                                                             "   7.1                                                                              0.45                                                                             0.08                                                                             2.0                                                                             0.011                                                                            1.60                                                                             2.00                                                                             3.60                                      __________________________________________________________________________

                                      TABLE 1                                     __________________________________________________________________________    Mechanical properties at room temperature (RT)                                Heat treatment: (last step always air cooling)                                a) Hot shaping, solution annealing and aging for Y hours at X°         C.,                                                                           b) Hot shaping, solution annealing and aging for Y hours at X° C.,     followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.                           Heat X  Y  Z.sub.1                                                                           X.sub.1                                                                          R.sub.m                                                                           R.sub.p0.2                                        Alloy No.                                                                           treatment                                                                          °C.                                                                       h  °C./h                                                                      °C.                                                                       N/mm.sup.2                                                                        N/mm.sup.2                                                                        A.sub.5 %                                                                         Z %                                                                              H.sub.V 30                             __________________________________________________________________________    1     a    730                                                                              8  --  -- 1020                                                                              552 37.0                                                                              44.0                                                                             280                                          a    730                                                                              14 --  -- 1042                                                                              592 33.5                                                                              47.5                                                                             271                                          b    730                                                                              8  15  595                                                                              1058                                                                              586 35.6                                                                              47.0                                                                             323                                          b    750                                                                              4  15  600                                                                              1117                                                                              661 38.0                                                                              48.0                                                                             307                                    6     a    730                                                                              8  --  -- 1082                                                                              655 38.0                                                                              51.0                                                                             302                                          a    750                                                                              8  --  -- 1130                                                                              669 29.0                                                                              39.0                                                                             311                                          b    750                                                                              4  15  600                                                                              1165                                                                              732 17.3                                                                              16.0                                                                             308                                          b    750                                                                              8  15  600                                                                              1177                                                                              740 22.0                                                                              22.0                                                                             334                                    7     a    730                                                                              8  --  -- 1063                                                                              672 37.0                                                                              51.0                                                                             313                                          a    750                                                                              8  --  -- 1171                                                                              749 30.0                                                                              31.0                                                                             331                                          b    750                                                                              4  15  600                                                                              1185                                                                              862  7.0                                                                               5.2                                                                             381                                          b    750                                                                              8  15  600                                                                              1247                                                                              844 17.5                                                                              15.0                                                                             372                                    __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Mechanical properties at 260° C.                                       Heat treatment: (last step always air cooling)                                a) Hot shaping, solution annealing and aging for Y hours at X°         C.,                                                                           b) Hot shaping, solution annealing and aging for Y hours at X° C.,     followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.                           Heat X  Y  Z.sub.1                                                                           X.sub.1                                                                          R.sub.m                                                                           R.sub.p0.2                                        Alloy No.                                                                           treatment                                                                          °C.                                                                       h  °C./h                                                                      °C.                                                                       N/mm.sup.2                                                                        N/mm.sup.2                                                                        A.sub.5 %                                                                         Z %                                                                              H.sub.V 30*                            __________________________________________________________________________    1     a    730                                                                              8  --  --  894                                                                              483 37.0                                                                              49.0                                                                             277                                          a    730                                                                              14 --  --  928                                                                              530 36.0                                                                              47.0                                                                             280                                          b    730                                                                              8  15  595                                                                               953                                                                              547 32.4                                                                              40.0                                                                             296                                          b    750                                                                              4  15  600                                                                              1003                                                                              621 32.0                                                                              49.0                                                                             327                                    6     a    730                                                                              8  --  --  984                                                                              575 36.0                                                                              46.0                                                                             308                                          a    750                                                                              8  --  -- 1043                                                                              605 32.0                                                                              35.0                                                                             305                                          b    750                                                                              4  15  600                                                                              1125                                                                              n.b.                                                                              15.0                                                                              19.0                                                                             345                                          b    750                                                                              8  15  600                                                                              1084                                                                              658 20.5                                                                              20.0                                                                             335                                    7     a    730                                                                              8  --  --  999                                                                              630 36.0                                                                              48.0                                                                             303                                          a    750                                                                              8  --  -- 1100                                                                              682 25.5                                                                              28.0                                                                             340                                          b    750                                                                              4  15  600                                                                              1096                                                                              909  3.0                                                                               5.0                                                                             381                                          b    750                                                                              8  15  600                                                                              1141                                                                              766 12.5                                                                              17.0                                                                             366                                    __________________________________________________________________________     *) = Hardness measurement performed at RT                                

                                      TABLE 4                                     __________________________________________________________________________    Mechanical properties at room temperature (RT)                                Heat treatment: (last step always air cooling)                                b) Hot shaping, solution annealing and aging for Y hours at X° C.,     followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,                    c) Hot shaping, water quenching, aging for Y hours at X° C.,           followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,                    d) as c), but with further controlled cooling from X.sub.1 with Z.sub.2       ° C./h to X.sub.2 ° C.                                          Alloy                                                                             Heat X     Z.sub.1                                                                           X.sub.1                                                                          Z.sub.2                                                                           X.sub.2                                                                          R.sub.m                                                                           R.sub.p0.2                                                                        A.sub.5                                                                          Z                                     No. treatment                                                                          °C.                                                                       Y h                                                                              °C./h                                                                      °C.                                                                       °C/h                                                                       °C.                                                                       N/mm.sup.2                                                                        N/mm.sup.2                                                                        %  %  H.sub.V 30                         __________________________________________________________________________    3   b    730                                                                              8  15  580                                                                              --  -- 1084                                                                              593 31.5                                                                             32.0                                                                             341                                    c    730                                                                              8  15  580                                                                              --  -- 1191                                                                              916 25.3                                                                             33.0                                                                             390                                    d    730                                                                              4   5  700                                                                              15  580                                                                              1166                                                                              8641                                                                              22.1                                                                             29.0                                                                             361                                    b    750                                                                              4  15  600                                                                              --  -- 1139                                                                              650 27.5                                                                             31.0                                                                             354                                    c    750                                                                              4  15  600                                                                              --  -- 1182                                                                              949 22.5                                                                             30.0                                                                             401                                    d    750                                                                              0.5                                                                               5  700                                                                              15  580                                                                              1143                                                                              820 23.6                                                                             31.0                                                                             368                                5   b    730                                                                              8  15  580                                                                              --  -- 1123                                                                              682 26.0                                                                             24.0                                                                             343                                    c    730                                                                              8  15  580                                                                              --  -- 1246                                                                              955 12.5                                                                             13.0                                                                             414                                    d    730                                                                              4   5  700                                                                              15  580                                                                              1071                                                                              625 31.0                                                                             30.0                                                                             298                                __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Mechanical properties at 260° C.                                       Heat treatment: (last step always air cooling)                                a) Hot shaping, solution annealing and aging for Y hours at X° C.,     followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,                    b) Hot shaping, water quenching, aging for Y hours at X° C.,           followed by controlled                                                        cooling with Z.sub.1 ° C./h to X.sub.1 ° C.                           Heat X     Z.sub.1                                                                           X.sub.1                                                                          R.sub.m                                                                           R.sub.p0.2                                        Alloy No.                                                                           treatment                                                                          °C.                                                                       Y h                                                                              °C./h                                                                      °C.                                                                       N/mm.sup.2                                                                        N/mm.sup.2                                                                        A.sub.5 %                                                                         Z %                                                                              H.sub.V 30*                            __________________________________________________________________________    3     b    730                                                                              8  15  580                                                                               980                                                                              540 34.0                                                                              43.0                                                                             321                                          c    730                                                                              8  15  580                                                                              1072                                                                              794 22.5                                                                              33.0                                                                             393                                          b    750                                                                              4  15  600                                                                              1002                                                                              569 28.0                                                                              38.0                                                                             359                                          c    750                                                                              4  15  600                                                                              1069                                                                              874 21.0                                                                              34.0                                                                             411                                    5     b    730                                                                              8  15  600                                                                              1084                                                                              593 31.5                                                                              32.0                                                                             341                                          c    730                                                                              8  15  600                                                                              1135                                                                              866 14.0                                                                              21.0                                                                             393                                          b    750                                                                              4  15  600                                                                              1139                                                                              650 27.5                                                                              31.0                                                                             354                                          c    750                                                                              4  15  600                                                                              1155                                                                              938 15.0                                                                              25.0                                                                             432                                    __________________________________________________________________________     *) = Hardness measurement performed at ET                                

                                      TABLE 6                                     __________________________________________________________________________    Mechanical properties at room temperature (RT)                                Heat treatment:                                                               c) Hot shaping, water quenching, aging for Y hours at X° C., then      controlled cooling                                                            with Z.sub.1 ° C. to X.sub.1 ° C., then air cooling                   Heat        Z.sub.1 R.sub.m                                                                           R.sub.p0.2                                      Alloy No.                                                                           treatment                                                                          X °C.                                                                      Y h                                                                              °C./h                                                                      X.sub.1 °C.                                                                N/mm.sup.2                                                                        N/mm.sup.2                                                                        A.sub.5 %                                                                         Z %                                     __________________________________________________________________________    2     c    730 4  15  580 1019                                                                              679 40.0                                                                              60.0                                          c    730 8  15  580 1083                                                                              863 32.0                                                                              49.0                                          c    750 4  15  600 1109                                                                              820 28.5                                                                              44.0                                    4     c    730 4  15  580 1108                                                                              822 29.0                                                                              44.0                                          c    730 8  15  580 1145                                                                              939 25.5                                                                              38.0                                          c    750 4  15  600 1154                                                                              912 24.5                                                                              32.0                                    __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Mechanical properties at 260° C.                                       Heat treatment:                                                               c) Hot shaping, water quenching, aging for Y hours at X° C.,           followed by controlled                                                        cooling with Z.sub.1 ° C. to X.sub.1 ° C.                             Heat        Z.sub.1 R.sub.m                                                                           R.sub.p0.2                                      Alloy No.                                                                           treatment                                                                          X °C.                                                                      Y h                                                                              °C./h                                                                      X.sub.1 °C.                                                                N/mm.sup.2                                                                        N/mm.sup.2                                                                        A.sub.5 %                                                                         Z %                                     __________________________________________________________________________    2     c    730 4  15  580  822                                                                              434 42/3                                                                              59.0                                          c    730 8  15  580  972                                                                              768 30.5                                                                              49.0                                          c    750 4  15  600 1046                                                                              693 24.0                                                                              48.0                                    4     c    730 4  15  580  929                                                                              635 37.5                                                                              48.0                                          c    730 8  15  580 1047                                                                              726 23.8                                                                              36.0                                          c    750 4  15  600 1056                                                                              802 18.8                                                                              36.0                                    __________________________________________________________________________

                  TABLE 8                                                         ______________________________________                                        Results of stress corrosion cracking tests                                    Solution A heated to 232° C.                                           Test load: 100% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        3     730    8      15    580  675   6      26 days/                                                                      no failure                                                             7      26 days/                                                                      no failure                                                             8      24 days/                                                                      no failure                              750    8      15    600  751   10     26 days/                                                                      no failure                                                             11     24 days/                                                                      no failure                                                             12     24 days/                                                                      no failure                        6     730    8      15    580  831   14     26 days/                                                                      no failure                                                             15     26 days/                                                                      no failure                              750    8      15    600  887   2      24 days/                                                                      no failure                                                             3      24 days/                                                                      no failure                                                             4      24 days/                                                                      no failure                        ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        Results of stress corrosion cracking tests                                    Solution A heated to 232° C.                                           Test load: 120% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        3     730    8      15    580  675   8      26 days/                                                                      no failure                        ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Results of stress corrosion cracking tests                                    Solution B heated to 232° C.                                           Test load: 100% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        3     750    8      15    600  751   12     23 days/                                                                      no failure                        ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        Results of stress corrosion cracking tests                                    Solution B heated to 232° C.                                           Test load: 120% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        3     730    8      15    580  810   8      25 days/                                                                      no failure                        ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        Results of stress corrosion cracking tests                                    Solution B heated to 260° C.                                           Test load: 100% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        2     730    8      15    580  780   2      24 days/                                                                      no failure                              750    8      15    600  763   5      25 days/                                                                      no failure                        3     730    8      15    580  683   26     24 days/                                                                      no failure                        4     730    8      15    580  772   8      24 days/                                                                      no failure                              750    8      15    580  756   6      25 days/                                                                      no failure                        5     730    8      15    580  748   34     24 days/                                                                      no failure                        ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                        Results of stress corrosion cracking tests                                    Solution B heated to 260° C.                                           Test load: 120% R.sub.p0.2                                                    Heat treatment: heat shaping, water quenching,                                aging for Y hours at X° C., followed                                   by controlled cooling with Z.sub.1 ° C./h to X.sub.1 ° C.,      then air cooling                                                                                             Test                                           Alloy X      Y      Z.sub.1                                                                             X.sub.1                                                                            load  Specimen                                 No.   °C.                                                                           h      °C./h                                                                        °C.                                                                         N/mm.sup.2                                                                          No.    Results                           ______________________________________                                        2     730    8      15    936  936   3      24 days/                                                                      no failure                              750    8      15    600  916   7      25 days/                                                                      no failure                        3     730    8      15    580  820   27     24 days/                                                                      no failure                        4     730    8      15    580  926   3      24 days/                                                                      no failure                              750    8      15    600  907   7      25 days/                                                                      no failure                        5     730    8      15    580  898   35     24 days/                                                                      no failure                        ______________________________________                                    

We claim:
 1. A process for the manufacture of structural componentswhich have very good resistance to corrosion and a 0.2% proof stress ofat least 500 N/mm², comprisinga) producing ingots from an alloy having43to 51% nickel 19 to 24% chromium 4.5 to 7.5% molybdenum 0.4 to 2.5%copper up to 1% manganese up to 0.5% silicon up to 0.02% carbon up to 2%cobalt
 0. 3 to 1.8% aluminium0.9 to 2.2% titanium,balance iron andincidental impurities, b) homogenizing said ingots at 1220° C. and thenhot shaping at a temperature above 1000° C. into components, followed byquenching said components in water, and c) precipitation hardening saidcomponents for 4 to 16 hours at 650° to 750° C., and then subjectingsaid components to air cooling.
 2. A process according to claim 1wherein said ingots are produced from an alloy having43 to 51% nickel 20to 23.5% chromium 5 to 7% molybdenum 1.5 to 2.2% copper up to 0.8%manganese up to 0.1% silicon up to 0.015% carbon up to 2% cobalt 0.4 to0.9% aluminium 1.5 to 2.1% titanium,balance iron and incidentalimpurities.
 3. A process according to claim 1 or 2, wherein after saidcomponents are quenched in water, said components are held for 4 to 10hours at 700°-750° C., then furnace-cooled by 150° C. at a rate of5°-25° C. per hour, and thereafter subjected to air cooling.
 4. Aprocess according to claim 1 or 2 wherein after said components arequenched in water, said components are held for 30 minutes at 730°-750°C., furnace-cooled to 700° C. at a rate of 5°-25° C. per hour and thento 580° C. at a rate of 2°-15° C. per hour, and thereafter subjected toair cooling.
 5. A process according to claim 1 or 2 further comprisingsolution annealing said components at 1,150° to 1,190° C. prior toquenching said components in water.
 6. A process according to claim 5wherein after said components are quenched in water, said components areheld for 4 to 10 hours at 700° to 750° C., then furnace-cooled by 150°C. at a rate of 5°-25° C. per hour, and thereafter subjected to aircooling.